Landfilling and landfill gas. Landfilling and dumping dominate waste disposal in the United States, as well as waste disposal worldwide. The US EPA estimates US landfilling of Municipal Solid Waste (“MSW”) at about 160 million tons annually over the past few years (U.S. EPA 2002) And, worldwide, even greater tonnages of organic solid wastes, severalfold those of the US, are landfilled and otherwise buried in dumps around the world
In wastes either buried in landfills or simply dumped, organic components decompose to form “landfill gas” (LFG) or biogas. LFG (biogas) comprises approximately equal volumes of methane and carbon dioxide, with lesser amounts of other gases and moderate levels of pollutants. A representative reaction for methane generation from cellulose, the largest fraction of most organic waste, is:
Importance of Landfill Gas Recovery.
The landfill gas (“LFG”) has pronounced environmental impacts, and its recovery is of extremely high importance across the United States (US), and worldwide, for a variety of related reasons:
1. Mitigating climate or “greenhouse” effect The climate or “greenhouse” effect of methane emitted from landfills is a major global concern, simply because of enormous amounts of organic wastes, hence methane emissions, involved. In climate terms, methane from landfills adds to (makes a difference of) 3 to 10% in the annual increase in radiative forcing due to buildup of all greenhouse gases in earth's atmosphere. In more simplified terms, the presence or absence of landfill methane emissions into the atmosphere can be considered to make a difference of about 3 to 10% in the “greenhouse effect”
2. Potential for renewable LFG energy The landfill methane recovered from landfills can be important as a fuel. It is usable with existing technology and equipment (Augenstein and Pacey, 1992). Available US landfill methane, potentially recoverable (as of now, based on the US Energy Information Agency (EIA) and other statistics) but not exploited for energy, conservatively equates to energy value of over 150,000 barrels of oil a day.
This amount of energy is significant in terms of improved national security and energy self-sufficiency for the US. Furthermore, because methane from wastes is nearly all from photosynthetically fixed carbon, this methane is a renewable fuel. It displaces the use of fossil carbon fuels, thereby lessening climate effects of fossil CO2.
3. Other important reasons for landfill gas capture. Other important reasons for landfill gas control and recovery include (a) need to mitigate methane effects on stratospheric ozone destruction, (b) prevent emission of local air pollutants, (c) mitigation of landfill methane migration and explosion hazards, and mitigation of odor problems, and (d) desires of the United States to develop practical, economic and cost-effective options for voluntary or non-voluntary greenhouse gas abatement actions, and practical options for carbon sequestration.
For all of these reasons, the recovery of landfill gas at high efficiency is a high priority to regulators as well as landfill owners and operators.
Notwithstanding the potential benefits of methane recovery, “conventional” landfill gas extraction, by the dominant EPA and state-accepted approach, is relatively inefficient.
Conventional Gas Extraction With Wells or Trenches.
The usual gas recovery approach is to use deep wells attached to a network of pipes and a gas pump (blower) that applies vacuum to extract the gas from waste. To illustrate performance of conventional systems, gas flow dynamics with “conventional” well (or trench) extraction are shown qualitatively in FIG. 1. FIG. 1 shows landfill 1 containing waste 2. A well 3 collects biogas from the landfill. Cover layers 4 are in contact with the atmosphere at the surface of the landfill. Arrows in FIG. 1 denote directions of gas fluxes, through (in and out of) a waste landfill surface, and within the waste. Gas flow velocity is denoted qualitatively by lengths of the arrows. Note the gas escaping to the atmosphere far from the wells. It is principally because of this LFG emission and loss far from the wells that gas capture is typically 60–85% (SWANA 1994. Solid Waste Association of North America Workshop on Landfill Gas Modeling and Recovery, 1994. Personal communications from participants). This inefficiency is acknowledged and estimated at 75% by the US EPA (EPA, Peer et al. 1991, ICF, 2002) and California Air Resources Board. The inefficiency has been an accepted feature of extraction.
The profile of surface emission flux is recognized to lead to potential for some emissions away from the wells under most circumstances. Note also that there is almost always entrainment of gas, whether LFG or atmospheric air, through the surface area most proximate to deep collection. Both LFG emission far from wells, and air entrainment proximate to subsurface collection, are well recognized as deleterious to collection efficiency. A “tradeoff” exists between extracting or “pulling” at too high a flow rate and entraining excessive atmospheric air, and pulling too little and recovering less LFG. This poses one dilemma of conventional extraction.
Geomembrane over highly conductive layer: Zison. An invention that partially ameliorates inefficiency and air entrainment problems of gas collection by wells has been to collect by a surface geomembrane or a low permeability layer over a surface or near-surface highly conductive layer. (Zison, U.S. Pat. No. 4,442,901). A schematic of the Zison highly conductive layer recovery method is shown in FIG. 2. In FIG. 2, biogas is emitted by digestion of waste 2. The arrows show flux of the gas. Overlying the bulk of the waste is a gas-permeable layer 5. A surface geomembrane 6 is used to prevent gas escape from the gas permeable layer. Biogas is extracted (arrow 21) from the gas permeable layer 5.
New landfill designs that facilitate collection of landfill gas (biogas), and new methods of collecting biogas generated in landfills are needed. Preferably the methods and designs will allow for more efficient collection of biogas than previous methods (i.e., allowing less biogas to escape). Preferably the methods and designs also minimize collection of atmospheric air with the biogas and minimize the drawdown of air into landfills. Air contamination harms the quality and utility of collected biogas, and air drawn into a landfill creates a more oxidizing environment in the landfill that leads to consumption of methane by oxidation and inhibits the anaerobic microbial fermentation that produces biogas.